Access to F125 is possible by various routes described in the literature, such as the fluorination of perchloroethylene or of one of its fluoro derivatives and the hydrogenolysis of pentachloroethane (F115). Pure F125 is a completely stable compound which does not undergo any transformation or decomposition in the normal conditions of synthesis, storage and use (T&lt;500.degree. C.). However, it is not easy to obtain an F125 of high purity and, regardless of the method of synthesis employed, the product contains, as main impurities, not only F115 but also various olefins which are more or less toxic and which, especially chlorotrifluoroethylene (F1113), are difficult to separate from F125.
Product purity specifications for F125 have been established from the toxicological study of F125 carried out as part of the PAFTT III (Program for Alternative Fluorocarbon Toxicity Testing). In the case of most of the olefins (F1111, F1113, F1114, etc), the specification for each of these is set according to its toxicity at a content of between 0 and 1000 ppm. Thus, in the case of the highly toxic olefins like PFIB (perfluoroisobutene), the permitted concentration is well below one ppb. Conversely, in the case of less toxic olefins, the specification is much less severe and can be up to several hundred ppm.
These specifications with regard to olefins are generally easy to meet when conventional processes for the synthesis of F125 (fluorination, hydrogenolysis) are employed. However, some of these olefins are difficult to separate from F125 merely by distillation, which is why their presence, and especially that of F1113, is often noted in the commercial products. At the present time, according to the suppliers, commercial F125 exhibits an F1113 content which oscillates between 5 and 90 ppm, in most cases between 20 and 80 ppm.
Until now the stabilization of HFCs has not seemed necessary and there has been concern only with that of the HCFCs (hydrochlorofluorocarbons) like 1,1- dichloro-1-fluoroethane (F114b) and 1,1-dichloro-2,2,2-trifluoroethane (F123). Thus, the stabilization of F141b containing traces of 1,1-dichloroethylene (F1130a) by means of an ethylenic hydrocarbon containing at least 4 carbon atoms has been described in patent FR 2 682 377; the use of .alpha.-methylstyrene or of nitromethane for stabilizing F141b is claimed in patent EP 539 719. To inhibit the degradation of F123 and/or of F123a when used in foams in the presence of polyols, patent EP 508 449 recommends the addition of nitrostabilizers like nitromethane. Application WO 92/17559 claims the stabilization of F123 by means of a phenol, of an aromatic compound or of an epoxide.